Direct tracking of H2 roaming reaction in real time

Roaming is an unconventional type of molecular reaction where fragments, instead of immediately dissociating, remain weakly bound due to long-range Coulombic interactions. Due to its prevalence and ability to form new molecular compounds, roaming is fundamental to photochemical reactions in small molecules. However, the neutral character of the roaming fragment and its indeterminate trajectory make it difficult to identify experimentally. Here, we introduce an approach to image roaming, utilizing intense, femtosecond IR radiation combined with Coulomb explosion imaging to directly reconstruct the momentum vector of the neutral roaming H2, a precursor to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{{{{\rm{H}}}}}}}}_{3}}^{+}$$\end{document}H3+ formation, in acetonitrile, CH3CN. This technique provides a kinematically complete picture of the underlying molecular dynamics and yields an unambiguous experimental signature of roaming. We corroborate these findings with quantum chemistry calculations, resolving this unique dissociative process.


COLTRIMS
To prove that the missing fragment D 2 in the incomplete channel, D + + C 2 N + , is indeed neutral, and not an ion that was simply lost in the detection process, we look at its kinetic energy as a function of pump-probe delay, shown in Supplementary Figure 2. The momentum vector and hence, the kinetic energy of D 2 are reconstructed by implementing momentum conservation among the three fragments.We observe a very low time-independent kinetic energy for D 2 which indicates that it is indeed a neutral dissociating from a charged fragment.On the contrary, the kinetic energy of an ion would typically decrease over time due to Coulomb explosion from the remaining charged moiety.The fact that acetonitrile has only 3 hydrogen atoms that can contribute to D + 3 formation makes it an ideal candidate to unambiguously track such roaming reactions.However, in order to be exhaustive in terms of all the possible ways that D + 3 can be formed in acetonitrile, we must also consider the possibility of a roaming neutral D which can abstract D + 2 from the remaining moiety to form D + 3 .For this, we have to consider the incomplete coincidence channel: D + 2 + C 2 N + + D. We note that the yield for this channel is nearly 4.5 times lower than D + + C 2 N + + D 2 , consequently making it a weak contributor to the D + 3 channel.This is not surprising, since a free H or D atom is a radical, which makes it less likely to be formed than H 2 or D 2 .In fact, this channel is not observed in our theoretical simulations due to the limited number of trajectories used in the present work.To our knowledge, it has also not been observed in previous works.Following the same sequence as before, we first ensure that the missing D is indeed a neutral by looking at its kinetic energy as a function of time-delay.Furthermore, in Supplementary Figure 3, we show the angular correlation between all pairs of fragments in the channel D + 2 + D neutral + C 2 N + .The broad and symmetric angular correlations between the neutral D and the other two ionic fragments are indications of roaming D. Unlike in Fig. 4 (c) of the main article, the correlation between D and C 2 N + is quite symmetric.This could be a result of D being a lighter roaming fragment compared to D 2 .In order to fully demonstrate the differences between a roaming and non-roaming neutral fragmentation channel, we have plotted in the left column of Supplementary Figure 4  ) = -0.643[1].A black dashed line of slope -0.643 is overlaid on this PIPICO channel to demonstrate the agreement between the expected and experimentally-obtained slope.Supplementary Figure 4 (b) shows the angular correlations between the momentum vectors of neutral H 2 O and C 2 H + 3 (blue line) and neutral H 2 O and CH + 3 (orange line).Both these distributions are asymmetrically shifted either towards or away from 0 • implying stronger momentum correlation, further corroborating that the neutral fragmentation of H 2 O proceeds through a secondary decay from C 2 H 5 O + .This behavior is in sharp contrast to that of corresponding angular distributions observed for the roaming neutral D 2 (shown to the right for comparison).Additionally, Supplementary Figure 4 (c) shows the Newton diagram for this channel over the first 200 fs pump-probe delay window.Here, the momentum vector of C 2 H + 3 is fixed along the x-axis, while those of CH + 3 and the reconstructed H 2 O are plotted on the top and bottom halves, respectively.Although the ionic fragments, C 2 H + 3 and CH + 3 , share similar characteristics to their counterparts in deuterated acetonitrile (shown in the right column of Supplementary Figure 4), the neutral fragments are significantly different.Specifically, the reconstructed momentum of neutral H 2 O is strongly directed towards C 2 H + 3 , which fully supports that H 2 O is produced through a secondary decay from C 2 H 5 O + .On the other hand, the reconstructed momentum of neutral D 2 displays no such preferential angularity and has a broad distribution centered at low momentum.Further insight into H 2 roaming has been obtained in a careful exploration of the potential energy surface (PES).In particular, we computed the minimum energy path (MEP) that a doubly charged acetonitrile molecule would follow after vertical ionization, i.e., the maximum gradient on the PES, starting from the optimized geometry of the neutral system (Supplementary Figure 6 (a)).We clearly observe the release of neutral H 2 , followed by the production of a weakly bonded H 2 ...HCCN 2+ complex.Furthermore, this shows that the neutral H 2 is polarized by the potential exerted by the dicationic fragment.We evaluated such potential by computing the electrostatic potential (ESP) of HCCN 2+ in two different configurations: using the geometry of the optimized structure from the moiety (Supplementary Figure 6 (b)) and using the geometry of the last point given in the MEP exploration (Supplementary Figure 6 (c)).The ESP is plotted in both cases by projecting the actual value on the electronic density isosurface, which provides a visualization of the potential that polarizes the neutral H 2 .Overall, Supplementary Figure 6 (b) and (c) reveal that the regions with the highest electrostatic potential (blue in color) are nearest to the C-H bond in HCCN 2+ .It is most likely that the roaming H 2 remains weakly-bound in this region.Upon fragmentation of H + and CCN + , the H 2 is approximately at 90 • with respect to the two ionic fragments, which qualitatively agrees with the angular correlations given in Fig. 4 (d A schematic of the experimental setup.II.SUPPLEMENTARY NOTE 2: RECONSTRUCTED KINETIC ENERGY OF D 2 NEUTRAL Supplementary Figure 2. Pump-probe delay dependent kinetic energy of the neutral D 2 from the channel D + + C 2 N + + D 2 .

Supplementary Figure 3 .
Angular distributions between pairs of fragments in the incomplete D+ 2 + C 2 N + + D channel propanol integrated over the first 200 fs.A black dashed line of slope -0.643 is overlaid on the PIPICO channel to show the agreement between the calculated slope and experimental coincidence channel.(b) Angular distributions between the momentum vectors of C 2 H + 3 and H 2 O (blue line), CH + 3 and H 2 O (orange line), CH + 3 and C 2 H + 3 (green line) within the first 200 fs time-delay window.(c) Newton plot for the channel CH + 3 + C 2 H + 3 + H 2 O neutral integrated over the first 200 fs of pump-probe delay.The momentum vector of C 2 H + 3 lies along the x-axis while those of CH + 3 and H 2 O neutral are plotted in the top and bottom halves, respectively.Fig. 4 from the main article is shown in the right column for comparison to D 2 neutral roaming dynamics in acetonitrile.
2 N + fit Supplementary Figure 5. (a) Kinetic energy release of D + 3 + C 2 N + as function of pump-probe delay.Projection of KER for (b) D + 3 + C 2 N + and (c) H + 3 + C 2 N + on the pump-probe delay axis.Red dashed lines obtained from the fit function (mentioned in the main text) are overlaid for comparison.The values from the fitting function are given in Supplementary Supplementary Figure 6.(a) Minimum Energy Path (MEP) followed in the potential energy surface of the doubly ionized acetonitrile; relative energy in eV referred to the neutral molecule as a function of the Intrinsic Reaction Coordinate (IRC).(b) and (c) Electrostatic potential (ESP) of HCCN 2+ mapped on the electronic density with isovalue 0.0004 a.u.. Color code for the extreme values: red = 0.32 a.u. and dark blue = 0.48 a.u..The geometry used in (b) corresponds to the channel H 2 /HCCN 2+ , i.e. after release of neutral H 2 , and in (c) to the weakly bonded H 2 ...HCCN 2+ , i.e. last point in the MEP.

8 .
Example of trajectory leading to H 2 emission.(a) Snapshots at different times.(b) Two C-H distances as a function of time: upper panel the H atom that remains bonded to the C atom ; lower panel one of the H atoms of the emitted H 2 .Newton plots for the channel D + + D + 2 + C 2 N + integrated over 200 fs time-delay window.The momentum vector of C 2 N + is fixed along the x-axis and the momentum vectors of D + and D + 2 are plotted in the upper and lower halves of the plot, respectively.
Supplementary Figure 14.Newton plots for the channel D + + D 2 + C 2 N + , integrated over 200 fs time-delay window, for four different time-windows: 0 -200 fs, 200 -400 fs, 400 -600 fs, and 600 -800 fs.The momentum vector of C 2 N + is fixed along the x-axis and the momentum vectors of D + and D 2 are plotted in the upper and lower halves of the plot, respectively.
the relevant dynamics from CH + 3 + H 2 O neutral + C 2 H + 3 in 2-propanol (CH 3 CHOHCH 3 ).The PIPICO channel, shown in Supplementary Figure 4 (a), is for CH + 3 measured in coincidence with C 2 H + 3 , with a missing mass of H 2 O.This channel involves two primary fragments, CH + 3 and C 2 H 5 O + , the latter of which subsequently dissociates into C 2 H + 3 and neutral H 2 O, with a slope that can be calculated

Table 1 :
Fit values